1. Field of the Invention
The present invention relates to a power supply and a control method thereof, and more particularly to a power supply and a control method thereof for reducing the number of switching times of a switching unit of an inverting part.
2. Description of the Related Art
FIG. 1 is a view illustrating a circuit of a conventional power supply for supplying AC power to a load 1. As shown in FIG. 1, the power supply comprises a rectifying circuit (not shown) to rectify a commercial AC power (110V/220V) to DC power, an inverting part 3 receiving the DC power rectified by the rectifying circuit, inverting the DC power to AC power having various frequencies and supplying the AC power to the load 1, such as a motor, and a switching controller 500 controlling switching units 11a, 11b, 21a and 21b of the inverting part 3 to open and to close.
The rectifying circuit generally comprises a diode rectifying circuit (not shown) rectifying the commercial AC power into DC power, and a capacitor (not shown) smoothing the DC power rectified by the diode rectifying circuit and transmitting the smoothed and rectified DC power to the inverting part 3.
The inverting part 3, which is connected to two ends of the rectifying circuit, receives the DC power rectified and smoothed by the rectifying circuit, then inverts the DC power to the AC power having the various frequencies and supplies the AC power having the various frequencies to the load 1.
The inverting part 3 comprises a full bridge circuit having a first bridge 10 and a second bridge 20 provided with respective pairs of switching units 11a and 11b, and 21a and 21b serially connected to each other. Diodes 13a, 13b, 23a and 23b, are respectively, connected to opposite ends of the switching units 11a, 11b, 21a and 21b of the first bridge 10 and the second bridge 20, the first bridge 10 and the second bridge 20 being in parallel. Tap nodes 14 and 24 between the respective pairs of switching units 11a and 11b, and 21a and 21b of the first bridge 10 and the second bridge 20 are connected to opposite ends of the load 1 and allow the AC power to be supplied to the load 1 by opening and closing of each of the switching units 11a, 11b, 21a and 21b. 
Transistors are used as the respective switching units 11a, 11b, 21a and 21b of the first bridge 10 and the second bridge 20. Gate ends of the transistors 11a, 11b, 21a and 21b are, respectively, connected to switching drivers 12a, 12b, 22a and 22b. The respective switching drivers 12a, 12b, 22a and 22b turn on and turn off the switching units 11a, 11b, 21a and 21b by transmitting voltages corresponding to logical values of switching control signals (AP, BP, AN and BN) outputted from the switching controller 500 to the gate ends of the transistors 11a, 11b, 21a and 21b. 
FIG. 2 illustrates an internal configuration of the conventional switching controller 500. As shown in FIG. 2, the switching controller 500 comprises a comparison signal generating part 132 outputting a voltage compared signal S by comparing a control voltage signal Vab transmitted from a control voltage signal generating part 140 with a comparison voltage signal VTRI transmitted from a comparison voltage signal generating part 150, a dead time generating part 134 allowing the voltage compared signal S to change and outputting a first switching signal SN and a second switching signal SP having dead times between each other. Further, the control voltage signal Vab outputted by the control voltage signal generating part 140 is a control signal to control electric power outputted to the load 1 through the inverting part 3, and is a sinusoidal waveform (refer to FIGS. 3A and 3B). Also, the comparison voltage signal VTRI outputted from the comparison voltage signal generating part 150 is a chopping waveform having a predetermined size and a predetermined period (refer to FIGS. 3A and 3B).
The comparison signal generating part 132 compares the control voltage signal Vab of the control voltage signal generating part 140 with the comparison voltage signal VTRI of the comparison voltage signal generating part 150. The comparison signal generating part 132 outputs a voltage compared signal S having a first logical value “1” when a magnitude of the control voltage signal Vab is greater than that of the comparison voltage signal VTRI, and outputs a voltage compared signal S having a second logical value “0” when the magnitude of the control voltage signal Vab is smaller than that of the comparison voltage signal VTRI.
The dead time generating part 134 changes the voltage compared signal S outputted from the comparison signal generating part 132 to the first switching signal SN and the second switching signal SP having the dead time between each other and outputs the first and second switching signals SN and SP. The dead time generating part 134 provides the first switching signal SN and the second switching signal SP having logical values opposite to each other. Further, a waveform of the first switching signal SN is an identical waveform to that of the voltage-compared signal S. Further, dead times are non-operating times set up to prevent a short circuit, which occurs when the switching units 11a, 11b, 21a and 21b of the first bridge 10 and the second bridge 20 are coincidently turned on.
The first switching signal SN outputted from the dead time generating part 134 is outputted as the switching control signals BP and AN of an upper arm switching unit 11a of the first bridge 10 and a lower arm switching unit 21b of the second bridge 20. The second switching signal SP outputted from the dead time generating part 134 is outputted as the switching control signals AP and BN of an upper arm switching unit 21a of the second bridge 20 and a lower arm switching unit 11b of the first bridge 10.
The switching control signals AP, BP, AN and BN outputted from the switching controller 500 are transmitted to the switching drivers 12a, 12b, 22a and 22b of the respective switching units 11a, 11b, 21a and 21b. The respective switching drivers 12a, 12b, 22a and 22b turn on and turn off the respective switching units 11a, 11b, 21a and 21b by applying voltages to the respective switching units 11a, 11b, 21a and 21b according to the inputted switching control signals AP, BP, AN and BN. Further, the lower arm switching unit 11b of the first bridge 10 and the upper arm switching unit 21a of the second bridge 20 are coincidently turned on by the switching control signals AP, BP, AN and BN. When the lower arm switching unit 11b of the first bridge 10 and the upper arm switching unit 21a of the second bridge 20 are coincidently turned on, the upper arm switching unit 11a of the first bridge 10 and the lower arm switching unit 21b of the second bridge 20 are coincidently turned off.
In a control method of the switching units 11a, 11b, 21a and 21b by the conventional switching controller 500, to supply AC power to the load 1, the respective switching units 11a, 11b, 21a and 21b of the first bridge 10 and the second bridge 20 are turned on and turned off corresponding to a waveform of the voltage compared signal S shown in FIG. 3B, such that switching operations are frequent, thereby causing a power loss.
Further, to remove heat generated due to frequent switching of the respective switching units 11a, 11b, 21a and 21b, a size of a heat sink is increased. Accordingly, cost of a power supply is increased.